A disk drive is a digital data storage device that stores data in substantially concentric tracks on the surface of a disk. During the operation of a disk drive, the disk is rotated at a substantially constant rate while data is written to or read from its surface. The disk drive is generally coupled to a host computer that delivers access requests to the disk drive whenever the host desires to store or retrieve digital information. To perform an access request, the disk drive first positions a head above the track of the rotating disk specified in the access request. Once the head is properly positioned, the requested data transfer (i.e., either a read operation or a write operation) is allowed to take place. During a read operation; data from the predetermined track is sensed by the head, after which it is processed by a read channel and delivered to the host system. During a write operation, data is received from the host, processed into a suitable format, and then delivered to the head which transfers the data to the predetermined track.
During operation of the disk drive, the head generally rides above the disk surface on a cushion of air (known as an air bearing) that is created by the movement of the disk under the head. The distance of the head from the disk while riding on the air bearing is referred to as the “flying height” of the head. To produce the “lift” required to hold up the head, head “sliders” are generally used that have the requisite aerodynamic qualities. In general, the performance of the disk drive will depend, to a great extent, on the maintenance of a proper head flying height. That is, performance will be degraded if the actual flying height of the head is considerably higher, for example, than a nominal flying height. This is particularly the case during write operations where an unexpected increase in flying height can result in written data that is unreadable.
A need therefore exists for a method and apparatus for determining whether the actual flying height of a head is within a desired range.